1. Field of the Invention
This invention provides a means for assuring a desired relative orientation of two or more sensors with respect to the gravitational vertical regardless of the rotational orientation of the housing in which the sensors are contained.
2. Discussion of the Prior Art
A sensor array intended for measurement of the gradient of a potential field such as the gravitational field, magnetic or electric field, or a pressure field as in seismic exploration, must assume some sort of identifiable orientation with respect to that field. For example, in marine seismic exploration, it is required to know the direction of arrival of seismic pressure waves at seismic sensors due to reflections from subsurface earth layers or from the air-water interface.
In seismic operations at sea, an acoustic source generates a wavefield that radiates outwardly in all directions by spherical spreading. A plurality of horizontally spaced-apart pressure sensors, such as hydrophones, are mounted in a long tubular-jacketed streamer cable at assigned stations therein. The streamer cable is towed in a substantially horizontal plane, at a selected depth, by a mother ship. The pressure sensors themselves are generally non-directional listening devices. If the seismic-wave propagation direction is of importance, then more than one sensor is required at each sensor station. Preferably at least two sensors disposed one above the other, are needed at every sensor station. If the orientation of the sensors with respect to the vertical is known accurately, the direction of acoustic-wave propagation can be determined.
One method has been proposed wherein two separate streamer cables are towed from the same ship, separated vertically from one another by several feet. Seismic streamer cables are very expensive, on the order of several hundred thousand dollars. Therefore such a method is decidely uneconomical. It is furthermore very difficult, if not impossible, to tow two streamers, several kilometers in length, so that they remain in a vertical plane and at a known separation over their entire length.
In another suggested method, a pair of sensors are fixedly mounted at each horizontally spaced-apart station inside the streamer cable jacket but on opposite sides of the jacket inner wall. Unfortunately, a seismic streamer cable twists and turns about its longitudinal axis when under tow so that the configuration of the sensor pair, relative to the vertical is never known.
It is well known that the sensors could be gimbal-mounted and balanced so that each sensor pair would always be vertical. But it has been found that gimbal mounting is too bulky and is too delicate to stand up under field conditions. Additional problems arise in connection with the slip rings and brushes that are needed to pick off the electrical seismic sensor signals.